An example of conventional cross-roller bearings is disclosed in, for instance Japanese Patent Laid-Open No. 1989-247818, which is of a type slim in thickness or height and includes more than one roller installed between an outside and inside races. The outside race is made therein with a cylindrical loading bore, which has an inside diameter somewhat larger than an outside diameter of the cylindrical roller but smaller than a diagonal dimension on an axial section of the roller, so that any roller dropped through the loading bore into a circular space between outside and inside raceway surfaces on the outside and inside races is allowed to fit snugly between the confronting raceway surfaces while rolling freely just after having made contact with any associated raceway surface. With the prior cross-roller bearing constructed slim in height as recited earlier, moreover, a cylindrical plug to close the loading bore is made such that the inside surface thereof is set back from the outside raceway surface to keep the plug against any engagement with the rollers after the plug has fit in the bore.
A swivel bearing with rolling-element loading plug is disclosed in Japanese Patent Laid-Open No. 1991-037414, which is of the sort of cross-roller bearing that is comprised of an outside race, inside race and more than one cylindrical roller interposed between the outside and inside races. A cylindrical loading bore is made in the inside race to insert the rollers into the space between the confronting races through there. A plug fits fixedly in the loading bore in such a way the inside end face thereof can merge smoothly with the inside raceway surface on the inside race. With the swivel bearing recited just above, the plug is made such that the inside end face thereof recedes somewhat from the raceway surface of the inside race after the plug has fit in the bore, while the raceway surface of the inside race is so made increasingly over any area before and after the loading bore as to make sure of smooth rolling of the cylindrical rollers across the plug.
Another type of prior roller bearings is disclosed in, for example U.S. Pat. No. 3,275,391, in which the circular space between an outside and inside races receives a plurality of cylindrical bearing rollers. A cylindrical loading plug fits in a corresponding bore, which is made in the outside race to insert the rollers into the space between the raceway surfaces through there. The cylindrical plug is fixedly secured in the associated bore with the radially outside end thereof sinking deep below the circular periphery of the outside race. The roller bearing is suited for use in a swivel bearing large in diameter, for example a matter of one meter in outside diameter, while the cylindrical plug is made apart from roller bearing to fit subsequently into the associated bore upon completion of assembly. The prior roller bearing constructed as recited just earlier is different than that in which the bearing fits entirely in any suitable housing, and also unlike the sort of the bearing construction in which the cylindrical plug is worked at the same time with internal working of the circular raceway in the race. The prior roller bearing constructed as recited just earlier does not fall in the sort where the rollers are under any prestressed condition.
The recently remarkable development in robotization technology in many industrial machines extensively requires any swiveling system using the cross-roller bearing that is high in stiffness to make to possible to turn smoothly without jarring even under high-speed revolution. To cope with the provision of high stiffness enough to allow the cross-roller bearing to turn with causing no jar or wobble, the prior art has heretofore developed many versions from making the clearance between any rolling element and its associated raceway surface as less as possible to applying high prestressed pressure to the rolling elements. Every prior version of the cross-roller bearings, nevertheless, requires several more chores including renewal of adjustment and working to serve both performances of high-speed turning with smoothness and high stiffness contributing to no jar or wobble.